Method for detecting network asymmetrical status and asymmetrical communication channels for power system

ABSTRACT

The present invention provides a new synchronization method named Multi-Node Echo Method (MNEM), which settles the problem of clock/data synchronization with asymmetrical channels. In MNEM, the echo messages are relayed among multi-node loop both by clockwise and anti-clockwise. Based on Multi-Node Echo Method, the invention further provides a method to detect symmetrical status of a communication network, a method to detect asymmetrical status of any channel in a communication network, and a method to calculate the sending and receiving delay values of any channel in a communication network, even when the channel is asymmetrical.

RELATED APPLICATION

This application is a national filing of PCT application Serial No.PCT/CN2009/000137, filed Feb. 9, 2009.

FIELD OF THE INVENTION

This invention relates to the field of power system, and moreparticularly to a Multi-Node Echo Method for detecting asymmetricalstatus of a power system, and further for detecting a communicationchannel's symmetrical or asymmetrical status. This invention can alsocalculate the delay of communication channel in some conditions.

BACKGROUND OF THE INVENTION

Along with the development and improvement of modern power communicationnetwork, self-healing ring network (SDH) routing is graduallyincreasing, which causes new problem to optical fibre currentdifferential protection for transmission lines.

Existing widely-used data synchronization technique for currentdifferential protection is echo method (ping-pong method) which assumesthat the sending and receiving delays are the same. Thus, once thecommunication routing changes, the delays of sending and receivingchannels may be no longer equal. As a result, the correlative clocks indifferent substations are no more synchronized, neither are thecorresponding data. It will severely affect the reliability ofprotection system and even lead to mal-tripping in some cases. Thus,clock synchronization or data synchronization with asymmetrical channelsis very important to the applications in power system, especially tocurrent differential protection.

JP2004226243A provides a solution for clock/data synchronization usingGPS, which will not be influenced by asymmetrical channels. GPS canprovide accurate, absolute and synchronized time for all substations intheory. But the GPS signal and GPS device are still not reliable enoughfor protection applications at present. As a result, GPS have not beenwidely used by current differential protections for synchronizationcurrently.

SUMMARY OF THE INVENTION

To overcome the shortcomings of present synchronization methods, a newsynchronization method named Multi-Node Echo Method (MNEM) is providedin this invention, which can detect asymmetrical status of network,detect asymmetrical status of a channel, and further calculate thechannel delays. Thus the present invention settles the problem ofdetecting asymmetrical status and/or channels of network and clock/datasynchronization with asymmetrical channels. It is more reliable andpractical using method of the present invention than using GPS. In MNEM,the echo messages are relayed among multi-node loop both by clockwiseand anti-clockwise. The channel in the present invention meanscommunication channel.

According to one aspect of the present invention there is provided amethod for detecting asymmetrical status of network with at least threenodes, comprising the steps of:

-   -   1) Select a set of echo loops which can cover all the existing        channels in the network;    -   2) Employ the multi-node echo method for each echo loop;    -   3) Detect and compare each echo loop's clockwise delay and        anti-clockwise delay. If each pair of the clockwise delay and        anti-clockwise are equal, it means all the loops are        symmetrical, thus, the whole network is in symmetrical status.        Otherwise, if any pair of delays is not equal, it means at least        one channel is asymmetrical; hence the network is in        asymmetrical status.

If it is a two-node network, one or more additional nodes can be addedto employ multi-node echo method.

In accordance with another aspect of the present invention there isprovided an alternative way of indicating the existence of asymmetricalstatus of a channel in network with at least four nodes. The methodcomprises the following steps:

-   -   1) Select two multi-node echo loops which both include the        channel to be tested;    -   2) Employ the multi-node echo method for the two echo loops;    -   3) Detect the symmetrical status of each echo loop by comparing        each loop's clockwise delay and anti-clockwise delay. If both        loops are asymmetrical, their common channel is asymmetrical.        Otherwise, their common channel is symmetrical.

If the network only contains three nodes, one or more additional nodescan be added to employ multi-node echo method.

In accordance with another aspect of the invention there is provided amethod for calculating the channel delays. The method comprises thefollowing steps:

-   -   1) Find out whether the channel is symmetrical or asymmetrical        using above method;    -   2) If the channel is symmetrical, calculate the delay with the        following steps:        -   a) Compute the sum of the sending and receiving delays of            the channel by traditional echo method; the result is            denoted by t_(sum);        -   b) Because the sending delay is equal to receiving delay of            the channel, each of them is the half value of t_(sum);    -   3) If the channel is asymmetrical, calculate the delay by MNEM        with the following steps:        -   a) Compute the sum of the sending and receiving delays of            the asymmetrical channel by traditional echo method; the            result is denoted by t_(sum);        -   b) Detect the difference of the sending and receiving delays            of the asymmetrical channel by MNEM, the result is denoted            by Δt, Δt is the value of subtracting the whole loop delay            which including the sending delay of the asymmetrical            channel from the whole loop delay which including the            receiving delay of the asymmetrical channel;        -   c) The sending delay of the asymmetrical channel is the half            value of the sum of t_(sum) and Δt.        -   d) The receiving delay of the asymmetrical channel is the            half value of the difference of t_(sum) and Δt.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter of the invention will be explained in more details inthe following description with reference to preferred exemplaryembodiments which are illustrated in the drawings, in which:

FIG. 1A to 1B shows traditional Echo Method for clock synchronization;in which, FIG. 1A shows a 2-node transmission line with communicationchannel, FIG. 1B shows channel delay of echo message;

FIG. 2A to 2C is multi-node echo method used in three nodes system; inwhich, FIG. 2A shows 3-node transmission lines with communicationchannels, FIG. 2B shows 3-node echo method loops, FIG. 2C shows channeldelay of echo message;

FIG. 3A to 3D is multi-node echo method used in four nodes system; inwhich, FIG. 3A shows 4-node transmission lines with communicationchannels, FIG. 3B shows 4-node echo method loops, FIG. 3C shows one pairof 3-node echo method loops, FIG. 3D shows another pair of 3-node echomethod loops;

FIG. 4A to 4C shows multi-node echo method according to 4-nodeembodiment of the invention; in which, FIG. 4A shows 4-node networkstructure, FIG. 4B shows MNEM loops for channel 2-4, FIG. 4C shows MNEMloops for channel 2-3.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Multi-Node Echo Method (MNEM) is different from traditional echo method(EM) both in process and situation.

The traditional echo method (EM) that is widely used in power systemcurrently is illustrated below for reference.

TABLE 1 Comparison between MNEM and EM Process Suitable scope EM Echomessage is transmitted Only symmetrical channels only between two nodesMNEM Echo message is transmitted Both symmetrical and among multi-node(≧3) loops. asymmetrical channels

FIG. 1 shows the principle of traditional echo method. The key point oftraditional echo method is to calculate the sending and receiving delaysof communication channel between two substations. As is shown in FIG. 1,an echo message is sent from substation-one 1 to substation-two 2 at thetime t_(s), substation-two 2 receives this message and sends it back tosubstation-one 1 after a delay of t_(d) that will be added into themessage. Finally, substation-one 1 receives the message (includes thevalues of t_(s), t_(e), and t_(d)) at the time t_(e). The delay of thechannel t can be calculated (suppose the sending and receiving channelare symmetrical, t₁₂=t₂₁).t=t ₁₂ =t ₂₁=(t _(e) −t _(s) −t _(d))/2  (1)

That the sending and receiving channels are symmetrical is prerequisitefor the traditional echo method, which may be influenced bycommunication route variation. The echo method will be invalid withoutthe prerequisite. It means, if t₁₂≠t₂₁, the equation (1) will not hold.

Multi-Node Echo Method

A new concept of Multi-Node Echo Method (MNEM) is proposed in thisinvention to resolve the problems. In MNEM, the echo messages arerelayed among multi-node loops both clockwise and anti-clockwise. MNEMwill bring more useful information about the delays of channels thantraditional echo method.

How to detect the symmetrical status of a three-node power system in useof MNEM is discussed below as an embodiment.

FIG. 2A shows a three-node (terminal) transmission line system. MNEM isemployed in this network. The echo message can be relayed alongdifferent loops. FIG. 2B shows a clockwise loop (1→3→2→1) and ananti-clockwise loop (1→2→3→1). Take the anti-clockwise loop of 1→2→3→1for example, its details are shown in FIG. 2C.

An echo message is sent from station-one 1 to substation-two 2 at timet_(s). Then, substation-two 2 receives the message and sends it tosubstation-three 3 after a delay of t_(d2). Substation-three 3 receivesthe message after a network delay of t₂₃, and sends it back tosubstation-one 1 after a delay of t_(d3). Substation-one 1 receives themessage finally at time t_(e), then the total delay of this echo loop(1→2→3→1) can be calculated by substation-one 1 as is shown below.t ₁₂ +t ₂₃ +t ₃₁ =C ₁ =t _(e) −t _(s) −t _(d2) −t _(d3)  (2)

Here t_(e), t_(s), t_(d2), t_(d3) can be got from the MNEM, they are allknown values when substation-one 1 finally receives the echo message.And then the total delay of anti-clockwise loop C₁ can be calculated.The total delay of clockwise loop (1→3→2→1) can also be calculated bythe same method below.t ₁₃ +t ₃₂ +t ₂₁ =C ₂  (3)

Here C₂ can be calculated by using MNEM.

The equations of (2) and (3) are the base of MNEM, which provide theoriginal data for subsequent steps of MNEM as exemplified further below.

MNEM will be used in a 3-node network and a 4-node network to illustratethe following method:

1) How to detect symmetrical status of a communication network. If oneor more channels become asymmetrical in the network, MNEM can detect theasymmetrical status which can give channel error alarm for currentdifferential protection and avoid mal-tripping. This method can beemployed by any network with at least three nodes.

2) How to detect asymmetrical status of any channel in a communicationnetwork. It can further point out the channel in asymmetrical status. Itwill bring much convenience for channel maintenance and repair. Thismethod can be employed by any network with at least four nodes

3) How to calculate the sending and receiving delay values of anychannel in a communication network, even when the channel isasymmetrical. It can be used for time compensation (datasynchronization). Current differential protection can work well evenwhen the channel is in asymmetrical status in this way. This method canbe employed by any network with at least four nodes.

Method One—Detect Asymmetrical Status of Network

As is shown in FIG. 2, there are two echo loops in three-node network.One is clockwise loop and the other is anti-clockwise loop. Thecorresponding total delays for the echo loops are shown below.t ₁₂ +t ₂₃ +t ₃₁ =C ₁  (4)t ₁₃ +t ₃₂ +t ₂₁ =C ₂  (5)

Here, C₁ and C₂ can be got by MNEM. If all the channels in this 3-nodenetwork are symmetrical, that is

$\begin{matrix}\left\{ \begin{matrix}{t_{12} = t_{21}} \\{t_{23} = t_{32}} \\{t_{31} = t_{13}}\end{matrix} \right. & (6)\end{matrix}$

As a result, the total delays of clockwise and anti-clockwise loops areequal (C₁=C₂). In power system, if the error between C₁ and C₂ is lessthan 0.1 ms, C₁ will be considered equal to C₂. This error margin mayvary according to different power system and different devices.

On the other hand, if C₁≠C₂, it means equation (6) will no longer hold.In other words, C₁≠C₂ means at least one of the channels in this networkis asymmetrical.

Therefore, asymmetrical status of network can be detected by comparingthe total delays of clockwise and anti-clockwise loops. If the delaysare equal, it means the network is in symmetrical status. Otherwise, itmeans at least one channel is asymmetrical; hence the network is inasymmetrical status.

This method can be employed for any multi-node network (node number ≧3).And a 4-node network case is illustrated below.

FIG. 3A shows a 4-node transmission line system with 6 communicationchannels. MNEM can be employed in this network to detect the symmetricalstatus.

There are lots of choices for the echo loops. The key point is that theset of loops should cover all the communication channels in the network.For the network in FIG. 3A, one available choice of loops is shown inFIG. 3B˜FIG. 3D. FIG. 3C˜FIG. 3D are 3-node echo methods which have beendiscussed before. FIG. 3B is a 4-node echo method which is similar to3-node one. The corresponding total delays for the echo loops are shownbelow.t ₁₂ +t ₂₃ +t ₃₄ +t ₄₁ =C ₁  (7)t ₁₄ +t ₄₃ +t ₃₂ +t ₂₁ =C ₂  (8)

Like the method used in a 3-node network, in 4-node network, if thetotal delays of clockwise and anti-clockwise loops are equal (C₁=C₂,here, if the error between C₁ and C₂ is less than 0.1 ms, C₁ will beconsidered equal to C₂), it means the network is in symmetrical status.If it is not equal (C₁≠C₂), it means at least one channel isasymmetrical; hence the network is in asymmetrical status.

In conclusion, for any multi-node network which contains at least 3nodes, the asymmetrical status of the network can be detected by thefollowing steps:

1) Select a set of echo loops which can cover all the existingcommunication channels in the network.

2) Employ the multi-node echo method for each echo loop.

3) Detect and compare each echo loop's clockwise delay andanti-clockwise delay.

-   -   If each pair of the clockwise and anti-clockwise delays is        equal, it means all the loops are symmetrical, thus, the whole        network is in symmetrical status. Otherwise, if any pair of        delays is not equal, it means at least one channel is        asymmetrical; hence the network is in asymmetrical status.

If it is a two-node network, one or more additional nodes can be added,and then the MNEM can also be employed.

If the network is detected to be in asymmetrical status, thedifferential protection can be disabled.

Method Two—Detect Asymmetrical Status of any Channel in Network

MNEM can further detect the asymmetrical channel based on method one. Itwill bring much convenience for channel maintenance and repair. Thismethod can be employed by any network with at least four nodes.

This method is based on the assumption that there is no more than onechannel asymmetrical at the same time in the restricted network (thepart of the whole network includes at least two loops and the two loopshave one common channel). The restricted network is always a smallnetwork and the probability that a channel becomes asymmetrical is verylow. Thus, there is no need to consider the case that more than oneasymmetrical channels exist at the same time in a small network.

If two loops in a network are detected both in asymmetrical status andthey have one common channel, the common channel is asymmetrical. Atypical case is shown below as a preferred embodiment.

As is shown in FIG. 4A, it's a 4-node network with 5 channels. The MNEMfor channel 2-4 is shown in FIG. 4B and that for channel 2-3 is shown inFIG. 4C.

As is shown in FIG. 4B, loop 1-2-4 (from substation-one 1 viasubstation-two 2 to substation-four 4 and back to substation-one 1) and2-3-4 have the common channel 2-4 and MNEM is employed in the two loops.If both loops are asymmetrical, it can be concluded that their commonchannel 2-4 is asymmetrical. Otherwise, if both loops are symmetrical orone loop is symmetrical and the other one is asymmetrical, it can beconcluded that the common channel is symmetrical. The four possibleresults of the MNEM in FIG. 4B are shown below.

$\begin{matrix}\left\{ \begin{matrix}{{t_{12} + t_{24} + t_{41}} \neq {t_{21} + t_{42} + t_{13}}} \\{{t_{23} + t_{34} + t_{42}} \neq {t_{32} + t_{43} + t_{24}}}\end{matrix}\Rightarrow{{Channel}\mspace{14mu} 2\text{-}4\mspace{14mu}{is}\mspace{14mu}{asymmetrical}} \right. & (9) \\\left\{ \begin{matrix}{{t_{12} + t_{24} + t_{41}} = {t_{21} + t_{42} + t_{13}}} \\{{t_{23} + t_{34} + t_{42}} = {t_{32} + t_{43} + t_{24}}}\end{matrix}\Rightarrow{{Channel}\mspace{14mu} 2\text{-}4\mspace{14mu}{is}\mspace{14mu}{symmetrical}} \right. & (10) \\\left\{ \begin{matrix}{{t_{12} + t_{24} + t_{41}} = {t_{21} + t_{42} + t_{13}}} \\{{t_{23} + t_{34} + t_{42}} \neq {t_{32} + t_{43} + t_{24}}}\end{matrix}\Rightarrow{{Channel}\mspace{14mu} 2\text{-}4\mspace{14mu}{is}\mspace{14mu}{symmetrical}} \right. & (11) \\\left\{ \begin{matrix}{{t_{12} + t_{24} + t_{41}} \neq {t_{21} + t_{42} + t_{13}}} \\{{t_{23} + t_{34} + t_{42}} = {t_{32} + t_{43} + t_{24}}}\end{matrix}\Rightarrow{{Channel}\mspace{14mu} 2\text{-}4\mspace{14mu}{is}\mspace{14mu}{symmetrical}} \right. & (12)\end{matrix}$

It is easy to prove the conclusion. Based on the power system's realconditions, the communication channels are symmetrical in mostconditions. It is a small probability that one communication channelbecomes asymmetrical. Thus, the present invention is base on theprecondition that there will be only one asymmetrical channel at thesame time. If both loops are asymmetrical, and their common channel 2-4is symmetrical, one can deduce that there are at least two asymmetricalchannels in the whole system. This contradicts with the precondition.

Furthermore, if there is a loop symmetrical, the common channel must besymmetrical. Otherwise both the two loops are asymmetrical.

There may be more loop choices, because there may be more than two loopswith the same common channel.

In conclusion, for any multi-node network with at least four nodes,method two can be achieved by the following steps.

1) For any certain channel to be tested in the network, select twomulti-node echo loops which both include the channel to be tested (thischannel is the common channel).

2) Employ the MNEM for the two echo loops.

3) Detect the symmetrical status of each echo loop by comparing eachloop's clockwise delay and anti-clockwise delay. If both loops areasymmetrical, their common channel is asymmetrical. Otherwise, theircommon channel is symmetrical.

It's obvious that there must be at least two multi-node loops with thesame common channel for method two of MNEM. Hence method two is onlysuitable for the network which has at least four nodes. For the networkthat has less than four nodes, additional nodes can be added and thenmethod two can be implemented.

Method Three—Calculate the Channel Delays

MNEM can be employed to calculate the channel delays accurately by anymulti-node network (≧4 nodes), even when the channel is asymmetrical. Itcan be used for time compensation (clock/data synchronization). Withmethod three, current differential protection can work well even whenthe channel is in asymmetrical status.

Method three is based on method two. After the asymmetrical channel isdetected by method two, the time difference of the sending and receivingdelays of this channel can be calculated. At the same time, traditionalecho method can also be employed in this channel to get the sum ofsending and receiving. These data can form an equation group, and then,sending and receiving delays can be calculated finally.

For example, in FIG. 4B, it is assumed that the channel 2-4 is detectedto be asymmetrical by method two and other channels are all symmetrical.It means,

$\begin{matrix}\left\{ \begin{matrix}{t_{24} \neq t_{42}} \\{t_{12} = t_{21}} \\{t_{14} = t_{41}} \\{t_{23} = t_{32}} \\{t_{34} = t_{43}}\end{matrix} \right. & (13)\end{matrix}$

The total delays of anti-clockwise loop (1→2→4→1) and clockwise loop(1→4→2→1) can be got by MNEM. They are (t₁₂+t₂₄+t₄₁) and (t₂₁+t₄₂+t₁₄).Then, the time difference of the sending and receiving delays of channel2-4 (Δt) can be calculated below:Δt=(t ₁₂ +t ₂₄ +t ₄₁)−(t ₂₁ +t ₄₂ +t ₁₄)=t ₂₄ −t ₄₂  (14)

Then, traditional echo method is employed in channel 2-4 and the sum ofsending and receiving delay t_(sum) is got below:t _(sum) =t ₂₄ +t ₄₂  (15)

The sending and receiving delays of this channel can be calculatedbelow:

$\begin{matrix}\left\{ \begin{matrix}{{\Delta\; t} = {t_{24} - t_{42}}} \\{t_{sum} = {t_{24} + t_{42}}}\end{matrix}\Rightarrow\left\{ \begin{matrix}{t_{24} = {\left( {t_{sum}\; + {\Delta\; t}} \right)/2}} \\{t_{42} = {\left( {t_{sum} - {\Delta\; t}} \right)/2}}\end{matrix} \right. \right. & (16)\end{matrix}$

In conclusion, for any multi-node network (node number N≧4), methodthree can be achieved by following steps.

1) Find the asymmetrical channel by method two of MNEM.

2) If the channel is symmetrical, calculate the delay with equation (1).

3) If the channel is asymmetrical, calculate the delay by MNEM withequation (14)˜(16).

It's obvious that there must be at least two multi-node loops with thesame common channel for method three of MNEM. Hence method three is onlysuitable for the network with at least four nodes. For the network withless than four nodes, additional nodes can be added and then methodthree can be implemented.

Though the present invention has been described on the basis of somepreferred embodiments, those skilled in the art should appreciate thatthose embodiments should by no means limit the scope of the presentinvention. Without departing from the spirit and concept of the presentinvention, any variations and modifications to the embodiments should bewithin the apprehension of those with ordinary knowledge and skills inthe art, and therefore fall in the scope of the present invention whichis defined by the accompanied claims.

The invention claimed is:
 1. A method for detecting an asymmetricalstatus of a communication network with at least three nodes for a powersystem, comprising: selecting a set of echo loops, wherein the set ofecho loops is selected by covering all the existing communicationchannels in the network; employing a multi-node echo approach for eachecho loop in the set of the echo loops by transmitting echo messages inboth clockwise and anti-clockwise direction through the echo loops;detecting and comparing each echo loop's clockwise delay andanti-clockwise delay; wherein if each pair of the clockwise delay andthe anti-clockwise delay are equal, the whole network is in symmetricalstatus; otherwise, the network is in asymmetrical status.
 2. The methodof claim 1, further comprising: adding, prior to steps of selecting,employing and detecting, an additional node if the network only containstwo nodes.
 3. The method of claim 1, further comprising: disablingdifferential protection in response to the network being in asymmetricalstatus.
 4. A method for detecting an asymmetrical status of a channel ina communication network with at least three nodes for a power system,comprising: selecting two multi-node echo loops, both including thechannel; employing a multi-node echo approach for each of the twomulti-node echo loops by transmitting echo messages in both clockwiseand anti-clockwise direction through the two multi-node echo loops;detecting and comparing each of the two multi-node echo loop's clockwisedelay and anti-clockwise delay; wherein if both pairs of the clockwisedelay and the anti-clockwise delay are unequal, a common channel of thetwo multi-node echo loops is asymmetrical; otherwise, the common channelof the two multi-node echo loops is symmetrical.
 5. The method of claim4, further comprising: adding, prior to steps of selecting, employingand detecting, an additional node if the network only contains threenodes.
 6. The method of claim 4, wherein the selected multi-node loopsare 3-node loops.